According to the federal Potency Monitoring Project, the average potency of marijuana has increased very little since the 1980s. Source: Quarterly Report #100, 104, University of Mississippi, National Center for Natural Products Research, NIDA Marijuana Project

Cannabis --
It is the most consumed drug in the world. All cannabis plants
contain delta-9 Tetrahydrocannabinol (THC), the main psychoactive
chemical component. THC potency varies from less than 1% to 20%
and over. During 1999, a total of 4,255 tons of cannabis (3,366
tons of herbal cannabis, 887 tons of cannabis resin and 2 tons
of cannabis oil) were seized against 3,442 tons in 1998 and 3,382
tons in 1997. Large cannabis seizures were made during the year
in Mexico, the US, Western Europe and Southern Africa.

Marijuana
and Medicine: Assessing the Science Base by Office
of National Drug Control Policy, Neuroscience
and Behavioral Health Institute of Medicine. Approved by the National Research Council, National Academy
of Sciences and the Institute of Medicine.

The
Marijuana Policy Project The Marijuana Policy Project is
the largest marijuana policy reform organization in the US.
MPP works to minimize the harm associated with marijuana. MPP
believes that the greatest harm associated with marijuana is
prison. MPP focuses on removing criminal penalties for marijuana
use, with an emphasis on making marijuana medically available
to seriously ill people who have the approval of their doctors.

(Abstract) Cannabis
use and cognitive decline in persons under 65 years of age.
This study investigates possible adverse effects of cannabis
use on cognitive decline after 12 years in persons under age
65 years.Concludes
cognitive decline occurs across all age groups with significant
proportions of all ages . Such decline is not associated with
cannabis use.Lyketsos,
Constantine G.; Garrett Elizabeth; Liang, Kung-Yee; Anthony
James C. American Journal of Epidemiology, Vol. 149, No.9,
1999